Field of the Described Embodiments
The described embodiments relate generally to internal combustion engines and to methods and arrangements for controlling internal combustion engines to operate more efficiently. More particularly, methods and arrangements for controlling internal combustion engines using variable valve lift and cylinder deactivation are described.
Description of the Related Art
The output of many internal combustion engines is controlled by adjusting the mass air charge (MAC) delivered to each fired cylinder. An engine control unit (ECU) directs delivery of the appropriate fuel charge for the commanded MAC. Gasoline fueled engines generally operate with an air/fuel ratio at or near stoichiometry to facilitate conversion of harmful pollutants to more benign compounds in a catalytic converter. Control of the MAC is most easily accomplished by adjusting the throttle position which in turn alters the intake manifold pressure (MAP). However, it should be appreciated that the MAC can be varied using other techniques as well. For example, variable intake valve lift control can be used to adjust the MAC. Adjusting the valve lift has the advantage of reducing pumping losses thereby increasing fuel efficiency, particularly at low or intermediate engine loads. A disadvantage of valve lift control is that the hardware needed to implement valve lift control tends to be relatively expensive and the control algorithms complex. Other techniques (e.g. altering the valve timing with a cam phaser) can also be used to adjust the MAC; however, use of a cam phaser has only a limited range of control over the MAC. There are a number of other engine parameters, including fuel charge, spark advance timing, etc. that may be used to alter the torque provided by each firing as well; however, use of these control parameters generally results in a lower fuel economy. If the controlled engine permits wide variations of the air-fuel ratio (e.g. as is permitted in most diesel engines), it is possible to vary the cylinder torque output by solely adjusting the fuel charge.
Over the years there have been a wide variety of efforts made to improve the fuel efficiency of internal combustion engines. One approach that has gained popularity is to vary the displacement of the engine. Most commercially available variable displacement engines effectively “shut down” or “deactivate” some of the cylinders during certain low-load operating conditions. When a cylinder is “deactivated”, its piston typically still reciprocates; however, neither air nor fuel is delivered to the cylinder so the piston does not deliver any net power. Since the cylinders that are shut down do not deliver any power, the proportional load on the remaining cylinders is increased, thereby allowing the remaining cylinders to operate with improved fuel efficiency. Also, the reduction in pumping losses improves overall engine efficiency resulting in further improved fuel efficiency.
Another method of controlling internal combustion engines is skip fire control where selected combustion events are skipped during operation of an internal combustion engine so that other working cycles operate at better efficiency. In general, skip fire engine control contemplates selectively skipping the firing of certain cylinders during selected firing opportunities. Thus, for example, a particular cylinder may be fired during one firing opportunity and then may be skipped during the next firing opportunity and then selectively skipped or fired during the next. This is contrasted with conventional variable displacement engine operation in which a fixed set of the cylinders are deactivated during certain low-load operating conditions. With skip fire control, cylinders are also preferably deactivated during skipped working cycles in the sense that air is not pumped through the cylinder and no fuel is delivered and/or combusted during skipped working cycles when such valve deactivation mechanism is available. Often, no air is introduced to the deactivated cylinders during the skipped working cycles thereby reducing pumping losses. The Applicants have filed a number of patent applications generally directed at dynamic skip fire control. These include U.S. Pat. Nos. 7,849,835; 7,886,715; 7,954,474; 8,099,224; 8,131,445; 8,131,447; 8,336,521; 8,449,743; 8,511,281; 8,616,181; and pending U.S. patent application Ser. Nos. 13/309,460; 13/654,244; and Ser. No. 13/654,248.
With skip fire control, certain firing patterns and/or firing fractions have been shown to provide preferred noise, vibration, and harshness (NVH) characteristics. Thus it is common to limit skip fire operation to a set of available firing patterns or firing fractions that have preferred NVH characteristics. However, limiting skip fire operation to a limited set of available firing patterns/fractions while operating the engine in an optimal manner may result in providing a mismatch between the output torque and the requested input torque command. This problem may be solved by changing engine manifold pressure through incrementally closing/opening the throttle blade or adjusting the cam phaser. Although these methods can provide a matched torque output to input torque command, they come at the expense of increased pumping losses.
Although conventional skip fire control works well to increase fuel efficiency, there are continuing efforts to even further improve engine efficiency.